Method of purifying propylene oxide

ABSTRACT

A process for purifying propylene oxide characterized by Subjecting a liquid reaction mixture containing propylene oxide, and water, hydrocarbons and oxygen-containing organic compounds as impurities obtained by reacting cumene hydroperoxide with propylene to extractive distillation using an extractant of a hydrocarbon having 7 to 20 carbon atoms with an extractive distillation column, and controlling a concentration of propylene glycol in the extractant supplied to the extractive distillation column to 20% by weight or less.

TECHNICAL FIELD

The present invention relates to a process for purifying propyleneoxide. More particularly, the present invention relates to anindustrially advantageous process for purifying propylene oxide bysubjecting a liquid reaction mixture containing propylene oxide obtainedby reacting cumene hydroperoxide with propylene to extractivedistillation using a hydrocarbon having 7 to 20 carbon atoms as anextractant, wherein the process can suppress deterioration of thecapability of the extractant and reduce the cost regarding theextractant.

BACKGROUND ART

As a producing process of propylene oxide, a process in which cumenehydroperoxide with propylene is reacted, is known.

In the liquid reaction mixture obtained by the reaction, water,hydrocarbons, oxygen-containing organic compounds such as methanol,propionaldehyde, acetone and methyl formate as impurities, are containedin addition to propylene oxide as a target product. Therefore,multi-stage purification steps become necessary to separate and recoverpropylene oxide of high purity from the liquid reaction mixture.

In purification of propylene oxide, use of a hydrocarbon as anextractant is publicly known. For example, U.S. Pat. No. 3,843,488discloses that an alkane such as octane is effective for removinghydrocarbons having 6 carbon atoms as impurities. Further, JP 50-007571B discloses that an alkane such as octane is effective for removal ofwater. Furthermore, U.S. Pat. No. 5,133,839 discloses that a hydrocarbonsuch as octane is effective for removal of impurities such as methanol,propionaldehyde and acetone.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide an industriallyadvantageous process for purifying propylene oxide by subjecting aliquid reaction mixture containing propylene oxide obtained by reactingcumene hydroperoxide with propylene to extractive distillation using ahydrocarbon having 7 to 20 carbon atoms as an extractant, wherein theprocess can suppress deterioration of the capability of the extractantand reduce the cost regarding the extractant.

That is, the present invention relates to a process for purifyingpropylene oxide, which comprises:

-   -   subjecting a liquid reaction mixture containing propylene oxide        obtained by reaction of cumene hydroperoxide with propylene, and        water, hydrocarbons and oxygen-containing organic compounds as        impurities to extractive distillation using a hydrocarbon having        7 to 20 carbon atoms as an extractant, with an extractive        distillation column, and    -   controlling a concentration of propylene glycol contained in the        extractant supplied into the extractive distillation column to        20% by weight or less.

BRIEF EXPLANATION OF THE DRAWING

FIG. 1 is a figure showing a flow of a purification process of thepresent invention.

EXPLANATION OF SYMBOLS

100: Extractive distillation column, 200: Extractive distillationcolumn, 300: distillation column

1. Line for supplying a propylene oxide raw material containing water,hydrocarbons and oxygen-containing compounds as impurities, 2. Bottomstream line of extractive distillation column (100), 3. Refluxing lineof extractive distillation column (100), 4. Overhead stream line ofextractive distillation column (100), 5. Extractant supplying line ofextractive distillation column (100), 6. Bottom stream line ofextractive distillation column (200), 7. Refluxing line of extractivedistillation column (200), 8. Purified propylene oxide stream line, 9.Extractant supplying line of extractive distillation column (200), 10.Refluxing line of distillation column (300), 11. Overhead stream line ofdistillation column (300)

BEST MODE FOR CARRYING OUT THE INVENTION

A raw material liquid to be subjected to the purification process of thepresent invention is a liquid reaction mixture containing propyleneoxide obtained by reaction of cumene hydroperoxide with propylene, andwater, hydrocarbons and oxygen-containing organic compounds asimpurities. Usually, the reaction is carried out in the presence of acatalyst under conditions under which the reaction temperature is 10 to200° C., the pressure is 1 to 20 MPa, and the amount of propylene is 2to 50 times, preferably 5 to 30 times by mole of that of cumenehydroperoxide. When the amount of propylene is below the above-describedrange, the reaction insufficiently proceeds or the yield of propyleneoxide decreases, on the other hand, when the amount of propylene isabove the above-described range, the production becomes disadvantageousin economical because of recovery cost and purge loss of unreactedpropylene. After propylene in the liquid reaction mixture has beenrecovered by distillation, the recovered propylene is recycled to thereactor again. The liquid reaction mixture containing propylene oxide(hereinafter, sometimes referred to as “liquid raw material”.) to besubjected to the purifying process of the present invention is obtainedby subjecting a reaction liquid after recovery of propylene to crudedistillation to separate propylene oxide from cumene and cumyl alcohol.

As hydrocarbons contained as the above-described impurities, saturatedand unsaturated hydrocarbons having 3 to 7 carbon atoms can be listed.

Further, as oxygen-containing organic compounds contained as impurities,aldehydes such as propionaldehyde, alcohols such as methanol, ketonessuch as acetone, carboxylic acid esters such as methyl formate, and thelike can be listed.

In the present invention, the liquid raw material is subjected toextractive distillation using a hydrocarbon having 7 to 20 carbon atomsas an extractant. As the extractant, linear saturated hydrocarbon suchas n-heptane, n-octane, n-nonane, n-decane, n-undecane and n-dodecane,branched saturated hydrocarbon such as 2,2-dimethylpentane,2,3-dimethylpentane, 2,2-dimethylhexane and 2,3-dimethyl hexane;unsaturated hydrocarbons thereof; and the like can be listed. Inaddition, these extractants can be used in any of a single and mixturethereof. From the industrial viewpoint, easily available saturatedhydrocarbons having 7 carbon atoms are preferable.

The extractive distillation is a distillation method adding anothercomponent as an extractant to a mixture containing two components ofwhich separation is difficult or impossible in usual distillationseparation thereby changing a relative volatility of the original twocomponents and making easy separation of the two components. Sincemethanol, water, acetaldehyde, hydrocarbons having 5 and 6 carbon atomsamong impurities in propylene oxide are near to 1 in a relativevolatility, these are difficult in separation by usual distillation. Forexample, in propylene oxide containing 0.1% by weight of methanol and0.1% by weight of 1-hexene, when respective relative volatilities ofthose to propylene oxide are compared, under atmospheric pressure, therelative volatility of methanol is 1.1 and that of 1-hexene is 1.1.Since both relative volatilities are near to 1, it is shown thatseparation by distillation of these is difficult. Accordingly, it isdifficult to industrially obtain propylene oxide having a sufficientpurity by the usual distillation separation method. In such this case,there is used a method which includes carrying out separation adding ahydrocarbon having 7 to 20 carbon atoms as an extractant to enlargerelative volatilities to propylene oxide of methanol, water,acetaldehyde and the hydrocarbons.

However, it was found that when propylene glycol in the hydrocarbonhaving 7 to 20 carbon atoms as an extractant, exists at a certainconcentration or higher, the effect of the extractant described abovewas markedly lost. In addition, the extractant is recycled, from theeconomical viewpoint, after separating propylene oxide and thenpurifying for separating impurities. However, when propylene glycol iscontaminated in the extractant, it is not separated from the hydrocarbonhaving 7 to 20 carbon atoms since it has a boiling point higher thanthose of methanol, water, acetaldehyde, hydrocarbons having 6 carbonatoms, and it was re-supplied into the distillation column together withthe extractant, accumulated and circulated. When it is re-supplied intothe extractive distillation column, the concentration of the extractantdecreases and the capability is deteriorated. Therefore, the amount ofthe extractant to be used increases and it lead to increase of energycost for purification of the extractant.

Propylene glycol easily produced in the co-existence of propylene oxideand water, since it is produced during production or purification ofpropylene oxide.

As a method of decreasing the concentration of propylene glycol,separation methods such as distillation, adsorption, washing, stillstanding separation and extraction, can be listed, but washing ispreferable from the economical viewpoint. For example, the concentrationof propylene glycol in the extractant can be easily reduced because theextractant and water occurs phase separation through washing of theextractant by adding water to the extractant, and propylene glycol ofwater-soluble is extracted to water phase.

According to the preferable embodiment of the present invention, theconcentration of propylene glycol in the hydrocarbon having 7 to 20carbon atoms as the extractant for purifying propylene oxide isdecreased to 20% by weight or less, preferably 10% by weight or less.Though the effect of the extraction becomes larger and the separationbecomes easier in inverse proportion of the concentration of propyleneglycol in the hydrocarbon having 7 to 20 carbon atoms as the extractant,when over 20% by weight, the effect of the extractant is killed andbecomes near the same as that in no extractant.

One example of a flow of preferable embodiment of the present inventionis shown in the drawing. A liquid raw material for purifying propyleneoxide, containing water, hydrocarbons and oxygen-containing organiccompounds as impurities is obtained by distilling and separating aliquid reaction mixture obtained through reaction of propylene withcumene hydroperoxide.

The liquid raw material is fed into an extractive distillation column100 through a line 1. At the same time, n-heptane is fed as anextractant. The extractive distillation column 100 is a column forremoving most of water and oxygen-containing organic compounds containedin the raw material, those are discharged as an overhead stream througha line 4, and as the bottom stream, n-heptane fed through a line 5 andpropylene oxide fed through the line 1 are obtained. In addition, a partof the overhead stream may be returned through the line 3. The bottomstream of the extractive distillation column 100 is fed to an extractivedistillation column 200 through a line 2, simultaneously n-heptane isfed to the extractive distillation column 200 through a line 9. Purifiedpropylene oxide is obtained through a line 8 from the overhead, most ofthe hydrocarbons contained in the raw material and n-heptane as theextractant are obtained through a line 6 from the bottom. Further, apart of the overhead stream maybe returned through a line 7. The bottomstream obtained from the extractive distillation column 200 is fed to adistillation column 300 through a line 6, most of the hydrocarbonscontained in the propylene oxide raw material are discharged through aline 11, and the bottom stream is recycled to the extractivedistillation column 100 and/or 200 through 5 and/or 9, respectively. Inthe flow, when propylene glycol is contaminated, the extraction effectfor removing the oxygen-containing compounds in the extractivedistillation column 100, is damaged, further in the extractivedistillation column 200, the extraction effect for removing thehydrocarbons, is markedly damaged. Furthermore, because propylene glycolhas a higher in a boiling point than propylene oxide or otherimpurities, propylene glycol is accumulated and circulated together withthe extractant, as the result, the volume of the extractant increasesleading to disadvantages in economical.

Accordingly, a decrease of the concentration of propylene glycol in theextractant is very effective. The place for removing propylene glycolmay be anywhere so far as the extractant exists, and it is preferable towash and remove propylene glycol near the extractive distillation column100 as a suitable place at which the effect of the present invention isput out at a maximum.

EXAMPLE

The present invention will be illustrated in detail referring toExamples below.

Examples 1 and 2, and Comparative Examples 1 and 2

When n-heptane was used as a extracting agent, comparisons of relativevolatilities under the equilibrium condition of propylene oxide, and2-methylpentane (hereinafter, 2 MP), methanol (hereinafter, MTA), waterand acetaldehyde (hereinafter, AA) as impurities, were carried out. Asconditions, under atmospheric pressure, the relative volatilities weremeasured by adding n-heptane as an extractant of three to four times byweight of propylene oxide (PO) containing the impurities describedabove, and the relative volatilities at the concentrations of propyleneglycol (hereinafter, PG) in n-heptane of 13% by weight and 36% byweight, respectively, ware measured and compared. Results are shown inTable 1. TABLE 1 PG concentration in extractant Relative volatility* (%By weight) 2MP MTA Water AA Comparative No use of extractant 1.5 0.7 0.91.5 Example 1 (Only PO containing impurities) Example 1  0 0.3 2.9 112.7 Example 2 13 0.3 1.2 1.2 1.3 Comparative 36 0.4 0.5 0.4 0.9 Example2*Relative volatility of each of components to PO

It is understood that though the relative volatilities of impuritiesbecome near 1 and usual separation is difficult when n-heptane is notused as an extractant, the volatilities of water, and AA and MTA as theoxygen-containing impurities become larger than that of PO and theseparation becomes easy. In addition, 2 MP as the hydrocarbon becomessmaller in volatility than PO, therefore, it is understood thatseparation of 2 MP together with the extractant from PO becomespossible.

However, it is understood that, as the concentration of PG becomeslarge, the relative volatility become near 1, and when the concentrationof propylene glycol becomes 36% by weight, with respect to 2 MP, theseparability decreases 1.5 times compared to 0 or 13% by weight of thePG concentration in the extractant, further, with respect to MTA, waterand AA, volatilities thereof become small though volatilities thereofbecome essentially large depending on the effect of the extractant,therefore the separation from PO becomes difficult when PO is recoveredfrom the overhead using the hydrocarbon as the extractant.

Accordingly, it is understood that in a case of extractive distillationusing a hydrocarbon as an extractant, when the PG concentration exceeds20% by weight, the effect of the extractant is markedly damaged.

INDUSTRIAL APPLICABILITY

As described above, according to the present invention, an process forpurifying propylene oxide by subjecting a liquid reaction mixturecontaining propylene oxide obtained by reacting cumene hydroperoxidewith propylene to extraction distillation using a hydrocarbon extractanthaving 7 to 20 carbon atoms, wherein the process for purifying propyleneoxide, is extremely advantageous from the viewpoint of an industrialoperation in particular, due to suppress deterioration in the capabilityof the extractant and reduce the cost regarding the extractant, can beprovided.

1. A process for purifying propylene oxide, which comprises: subjectinga liquid reaction mixture containing propylene oxide obtained byreaction of cumene hydroperoxide with propylene, and water, hydrocarbonsand oxygen-containing organic compounds as impurities to extractivedistillation using a hydrocarbon having 7 to 20 carbon atoms as anextractant, with an extractive distillation column, and controlling aconcentration of propylene glycol contained in the extractant suppliedinto the extractive distillation column to 20% by weight or less.
 2. Theprocess according to claim 1, wherein the extractant is recycled.
 3. Theprocess according to claim 2, the concentration of propylene glycol inthe extractant is controlled to 20% by weight or less by washing theextractant with water.
 4. The process according to claim 3, theconcentration of propylene glycol in the extractant is controlled to 10%by weight or less by washing the extractant with water.
 5. The processaccording to claim 1, the concentration of propylene glycol in theextractant is controlled to 20% by weight or less by washing theextractant with water.
 6. The process according to claim 5, theconcentration of propylene glycol in the extractant is controlled to 10%by weight or less by washing the extractant with water.